The present invention relates generally to combiners, and more particularly, to a blind coherent combiner and signal processing method employing coupled phase-lock loops.
The operation of coherently combining of multiple copies of a signal has many applications in communications systems. Beam-forming and diversity combining are two examples. Most adaptive algorithms used to obtain the combining weights require knowledge of the desired signal (i.e., a training sequence). This is discussed by Robert Monzingo and Thomas Miller in xe2x80x9cIntroduction to Adaptive Arraysxe2x80x9d, Wiley Interscience, 1980, for example.
However, in many applications, the use of training sequences is undesirable or impossible. Conventional blind beamforming schemes have been proposed in the literature. This is discussed by Theodore Rappaport, in xe2x80x9cSmart Antennas: Adaptive Arrays, Algorithms, and Wireless Locationxe2x80x9d, IEEE, 1998. Such conventional blind beamforming schemes tend to be computationally involved.
It is therefore an objective of the present invention to provide for a blind coherent combiner and signal processing method employing coupled phase-lock loops that overcomes limitations of conventional blind beamforning schemes.
To accomplish the above and other objectives, the present invention provides for a simple coherent combining scheme that does not require training to adapt the combining weights. The approach employed in the present invention uses phase-lock loops to demodulate input signals. The phase-lock loops are coupled so that their outputs are phase coherent. The outputs of the phase-lock loops are summed to obtain a coherent combining of the input signals. This approach is also advantageous in that it introduces little additional hardware into receivers that may employ it. This combining scheme may be implemented in either an analog or all-digital implementation.
Exemplary apparatus comprises two or more phase-lock loops having signal inputs and I and Q (in-phase and quadrature) data outputs. A combiner sums the I and Q data output by the two or more phase-lock loops. A common decision circuit feeds back the summed output of the combiner to the two or more phase-lock loops and that outputs phase-coherent output signals from the apparatus.
The signal processing method is used to couple two or more phase-lock loops that each output I and Q data to produce a phase coherent output signal. The method comprises the following steps. The I and Q data output by the two or more phase-lock loops are summed. The summed I and Q data are processed using a common decision circuit to generate a phase-coherent output signal. The summed I and Q data are fed back to the two or more phase-lock loops.
The present invention provides for coherent combining of digital communications signals. The approach of the present invention utilizes a novel coupling of phase-lock loops. This approach is simple, requires little additional hardware or processing, and operates without the use of training sequences.